<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var m = 1;

        function fun() {
            m++;
            var m = 3;
            console.log(m); //3
        }

        fun();
        console.log(m); //1
    </script>
</body>

</html>